It is known in the prior art to package a micromachined (or “MEMS”) device in a cavity package. Cavity packages are attractive for MEMS devices because they include an internal “cavity” that encloses the MEMS device without physically contacting or restraining a moveable portion of the MEMS device. The cavity area mainly protects the MEMS device from external stresses originating from thermal, torque and pressure loads. Although cavity packages are significantly reliable, they suffer from high cost.
Overmold packaging, while common for packaging non-micromachined integrated circuits, has presented challenges to MEMS packaging. The process of encapsulating a MEMS device may involve physical and thermal shock to the MEMS device. In addition, the overmold material properties widely change with temperature. In the case of a silicon-based MEMS device encapsulated in plastic overmold, this includes both the plastic's stiffness and thermal expansion coefficient, which are largely different from the corresponding properties of silicon. As a result, thermal stresses in the package due to the wide operational temperature, which may range for example from 175 C to −40 C, create large stresses that physically propagate through the structures in the MEMS sensor, and may cause performance problems, such as large offset drift of these sensors over the temperature.